I want to make the pack for Minecraft 1.6.2. I'm currently experimenting with MultiMC5 and a 1.6.2 version of HappyDiggers AMP and so far it seems to be working ok-ish.
cc is talking to someone about maybe making a custom launcher for us, at least untill we can get onto a 1.6 launcher for FTB. If that doesn't pan out we could use MultiMC5.
As for the extra dimension to add to RPG, I'm going to add the Erebus mod. We're a few of the lucky ones who have permission to use it in both AMP and RPG. The Erebus dimension is pretty scary and ads a ton of cool mobs to fight. Check it out here: http://www.minecraftforum.net/topic/1652014-